Pagewide printing dramatically increases print speeds compared to traditional scanning printheads. Many types of pagewide printers use overlapping printhead segments to form a pagewide array configured for printing over a required media width. For example, the Applicant's wideformat pagewide printers described in WO2011/011824 use a staggered overlapping array of multi-color A4-sized printheads, which extend across wideformat media. The Applicant's modular printing systems described in U.S. Provisional Application No. 62/330,779 filed 2 May 2016 use multiple monochrome pagewide print bars, each print bar having a staggered overlapping array of monochrome printheads (“print modules”). Other pagewide printers, such as those described in U.S. Pat. No. 8,662,636 use a staggered overlapping array of printhead chips to form a pagewide printhead. In the present context, the term “printhead segment” encompasses both printhead chips and printheads (or print modules) comprised of multiple printhead chips.
A problem with overlapping printhead segments is that pairs of neighbouring printhead segments must be stitched together in their overlap regions. As used herein, the term “stitching” refers to any method by which image content is printed from a pair of overlapping printhead segments. Whatever stitching method is used, print artifacts generally result from the stitching, either in the form of a light or dark streak down the page.
In its simplest form, butt joins may be used for stitching, whereby one printhead segment prints image content on one side of a fixed stitch point and a neighbouring overlapping printhead segment prints image content on the other side of the fixed stitch point. Test prints may be used to select the position of the fixed stitch point in the overlap region and thereby provide the least objectionable print artifacts.
More sophisticated methods for hiding stitch points have also been described in the art. For example, EP-A-0034060 describes a method of varying the position of the stitch point for overlapping printhead segments. In different lines of print, the stitch point may be varied in a random or cyclic manner in order to hide stitching artifacts.
U.S. Pat. No. 7,331,646 describes a stitching method whereby printing is shared between the overlapping printhead segments in the overlap region. Typically, printing is faded from one printhead to the other across the overlap region, in accordance with a dedicated overlap dither. This type of is stitching is sometimes referred to in the art as “feathering”.
The prior art has generally focused on various types of overlap dither in an attempt to improve stitching and minimize print artifacts. However, a problem with all stitching techniques is that the relative positioning of the overlapping printhead segments is never precisely known and, moreover, cannot be controlled to the necessary degree of accuracy during printhead manufacture or printhead replacement. A further problem is that two dots printed on top of each other typically produce a higher optical density than simply printing the same two dots separately. Therefore, producing a predictable optical density in the overlap region is virtually impossible, because overlapping dithers from overlapping printheads will produce an unpredictable number of dot-on-dot artifacts depending on the precise registration of the printheads. Hence, overlap dithers tend to produce a dark streak down the page when dot-on-dot printing occurs in the overlap region. Surprisingly, unsophisticated butt joins often produce the best results for many types of image content due to the above-mentioned problems with more sophisticated dithering methods. Nevertheless, butt joins are far from satisfactory when printhead segments are misaligned and/or somewhat rotated relative to each other.
With the commercial growth of pagewide printers having overlapping printheads and/or printhead chips, and particularly in view of customers' sensitivity to print quality in many market segments, it would be desirable to improve methods of stitching so as to reduce noticeable stitching artifacts and improve overall print quality.